As a technique of this kind, an engine lag down control system has been proposed to date. This engine lag down control system is to be arranged on hydraulic construction machinery, which has an engine, a variable displacement hydraulic pump, i.e., main pump driven by the engine, a swash angle control actuator for controlling the swash angle of the main pump, a torque regulating means for regulating the maximum pump torque of the main pump, for example, a means for controlling the swash angle control actuator such that the above-described maximum pump torque is held constant irrespective of changes in the delivery pressure of the main pump, a solenoid valve for enabling to change the maximum pump torque, a hydraulic cylinder, i.e., hydraulic actuator operated by pressure fluid delivered from the main pump, and a control lever device, i.e., control device for controlling the hydraulic actuator.
The conventional engine lag down control system is constituted by a processing program stored in a controller and an input/output function and computing function of the controller, and includes a torque control means and another torque control means. When a non-operated state of the control device has continued beyond a predetermined monitoring time, the former torque control means outputs a control signal to the above-described solenoid valve to control a maximum pump torque, which corresponds to a target number of engine revolutions until that time, to a predetermined low pump torque. In the course of the control by the torque control means, the latter torque control means holds the above-described predetermined low pump torque for a predetermined holding time subsequent to the operation of the control device from the non-operated state.
According to this conventional technique, upon quick operation of the control device from the non-operated state, the maximum pump torque is held at the predetermined low pump torque until the holding time elapses. At the time of a lapse of the holding time, the maximum pump torque is immediately changed to a rated pump torque, that is, the maximum pump torque corresponding to the target number of revolutions of the engine. During the holding time, the maximum pump torque is controlled at the predetermined low pump torque to reduce the load on the engine. Therefore, an engine lag down is controlled, in other words, a momentary reduction in engine revolutions when a sudden load is applied to the engine is controlled relatively small, thereby realizing the prevention of adverse effects on working performance and operability, a deterioration of fuel economy, an increase in black smoke, and the like (for example, see JP-A-2000-154803, Paragraph Numbers 0013, and 0028 to 0053, and FIGS. 1 and 3).